1. Technical Field
Described herein is a programmable logic circuit.
2. Related Art
The programmable logic circuit (also referred to as PLA (Programmable Logic Array)) is a rewritable basic logic circuit used in a large number of applications such as multiplexers, decoders, comparators or state machines.
In the conventional circuit utilizing the PLA such as a CPLD (Complex Programmable Logic Device), the main current is a circuit utilizing a storage device having a charge storage structure of floating gate (hereinafter, also referred to as FG) type in a transistor, such as the EPROM (Erasable Programmable Read Only Memory), EEPROM (Electronically Erasable and Programmable Read Only Memory) or a NAND flash memory. Such a circuit has been utilized in many applications (see, for example, U.S. Pat. No. 4,617,479).
As the transistors become finer, however, problems described below are posed. As the transistors become finer, the floating gate is also made finer. In many cases, floating gates having the same size in gate length are formed. As a result, the charge amount stored in the floating gate also decreases and dispersion of the stored charge is posing a problem.
Since charge stored in the floating gate decreases as a stochastic phenomenon even when it is retained, there is also a problem that the retention time decreases as the floating gate becomes finer. In other words, the finer floating gate brings about a negative effect in writing and retaining the charge. It is also pointed out that the tolerance is decreased and the number of times of rewriting is also limited by making various insulation films thinner at the same time. Typically, an error correction circuit is needed. If the error rate is increased, however, then it becomes impossible to do without simple error correction and a large-scale circuit becomes necessary, and consequently the effect of the finer floating gate might be canceled.
On the other hand, for writing charge into an element (FG type element) having a floating gate, a high voltage of at least 10 V is needed, resulting in a bad influence upon the circuit load and reliability. Furthermore, since the floating gate uses charge, there is a problem that it has a weakness for cosmic rays and software errors are apt to occur. This means that utilization of the FG type device serving as a storage element is limited in the case where LSIs are utilized in situations where a mistake causes a serious accident in an ultimate natural environment such as nuclear facilities or the cosmic space.